The invention relates to a method for preparation of test bodies for analysis of porous, preferably thermally sprayed, surface layers which are incorporated by casting in plastic.
To be able to analyze thermally sprayed surface layers, so called test bodies have to be produced, which are lapped and examined and evaluated with the use of a microscope with regard to the structure of the surf ace layer. It is when the test body is lapped that the problem arises in that the surface layer is “smeared out”, which will give a false structure.
There are two known methods for incorporating by casting of test bodies, viz. on one hand a cold casting method and on the other hand a warm casting method, which will be described below.
In the cold casting method, test bodies are produced by incorporating by casting a test piece under vacuum in a specific two-component resin of epoxy type with very good penetration ability into porous layers, i.e. the surface of the test pieces are wetted very well. Since the epoxy resin will fill out all pin-holes and voids in the surface layer, said surface layer can be lapped after curing the epoxy resin with a very good result without any smearing of said surface layer, whereby a true structure is obtained which can be evaluated with use of a microscope.
This method suffers from the drawbacks that on one hand the curing time for the epoxy system used at room temperature amounts to several hours and that on the other hand the epoxy resin used is very expensive, which means that the costs can be very high at a large number of tests.
In the other method, i.e. the hot casting method, test bodies are produced in the way that a sample of the surf ace layer is incorporated by casting in a pulverized phenolic plastic, preferably bakelite, at a pressure of about 5–15 kN and a temperature of about 150° C. in a so called Pronto-press. This method has the advantage that the incorporation by casting is very fast, about 15 minutes, and that the cost per body is very low. However, the drawback is that the bakelite does not penetrate into the pin-holes and voids in the porous surface layer, whereby the result of the lapping of the surface layer is non-useable, since there is a great risk for “smearing ” of the surface layer, which will produce a false structure.